The present invention relates to large-scale electrolysis furnaces of the type providing a heating current between a cathode and an anode.
The cathode of a conventional large-scale electrolysis furnace is known to consist of carbon blocks, all of the same electrical conductance, in which steel bars are embedded to conduct the electric current. The electrolysis current path extends from the anode, vertically through the electrolyte and, when passing into the molten aluminum layer covering the carbon cathode blocks, which layer has an electrical conductivity 2300 times better than the carbon blocks, is directed toward the side walls of the electrolysis cell, i.e. toward the point where the steel cathode bars emerge from the cathode blocks. As a result, the bottom of the electrolysis furnace directly underlying the anodes in the area of the longitudinal center axis of the furnace becomes electrically and thermally understressed whereas the region along the longitudinal sides of the furnace becomes electrically and thermally overstressed.
It is also known that aluminum oxide can be introduced into the electrolysis furnace along the longitudinal sides of the furnace as well as in the region of the longitudinal center axis. For an electrolysis furnace which is fed at the longitudinal sides, the thermal overstress in the region of the longitudinal sides of the furnace is compensated by the heat of solution absorbed by the dissolution of the aluminum oxide in the cryolite so that a temperature drop results from the center of the furnace to the longitudinal sides of the furnace. At the longitudinal sides of the furnace there thus is formed a crust of hardened electrolyte which protects the carbon side walls of the furnace against the corrosive molten electrolyte. The thickness of the crust depends on various parameters, for example the quantity of aluminum oxide introduced per unit time. Without the above-mentioned protective layer, the electrolysis operation cannot be continued for extended periods of time.
In electrolysis furnaces in which the aluminum oxide charges are fed in along the center longitudinal axis of the furnace this increase in the cathode current density has its full effect along the longitudinal sides of the furnace; in contradistinction to the above-mentioned feeding of the aluminum oxide charge at the longitudinal sides, this type of charging produces a temperature drop from the longitudinal sides of the furnace to the center axis of the furnace.
This means that a furnace which is charged with aluminum oxide along its center will have a higher temperature in the region of the longitudinal sides of the furnace than in the region of the center axis of the furnace since there is no absorption of heat of solution along the sides. A protective layer of hardened electrolyte can thus not form, or can form only with difficulty, along the longitudinal sides of the electrolysis furnace, which sides are of carbon.